Two-way telephone system



Oct. 19, 1943. u. s. BERGER TWO-WAY TELEPHONE SYSTEM Filed July 16. 1942 INVENTOR u S. BERGER awe 25W ATTORNEY Patented Oct. 19, 1943 um'reo s ares an ENT FF V 2,332,430 1 I Two-WAY TELEPHONE SYSTEM j Uriah $.Berger, New YorlnN. Y., assignor to Bell Telephone Laboratories, Incorporated,- New York, N 3! a corporation of New York I H Application July 1c, 1942, warm. 451,112 j 8 Claims (01. 179-81 The invention relates to two-way telephone transmission systems and particularly to circuits for controlling the directionand volume of signal transmission in such systems. I

The invention is particularly. applicable to, although not limited to, two-way telephone opcraters circuits or sets. It has been found that for certain conditions of use, such as centralized information and intercept service,,or-in locations Where there are very long subscribers loops, high room noise, poor sidetone balance. and low received speech levels combine to adversely affect service performance of the usual invariable antisidetone operators telephone circuit. v

to improve the An object of the invention is operation of such circuits.

Another object is to make an operators tele- 1 phone circuit more flexible, that is; to adapt it better for a variety of conditions. encountered in service. I l

Another object is to provide higher sensitivity and reduced susceptibility .toroom noise operation in an Operators telephone circuit.

Another object is to provide higher receiving efficiency in an operators telephone circuit without increasing the sidetone transmission.

These objects are attained in accordance with the invent-ion by the use withsuch circuits of I branches each including a detector followed by a direct current amplifier, is responsive to the voice currents of theoperatorwhen he talks'to properly control the loss values of the variolossers so as to switch loss of the proper value and at the proper rate out of the transmitter:

circuit and into the receiver circuit, to provide the desired'operation. i l r A feature of the invention is the use inflthe detector load circuit of each control branch, of

a varistor shunted by a condenser, operating to control in the proper ratio the rate of change in gain of the transmitter and receiver circuits provided by the vario-lossers, for the purpose of reducing sidetone transients as Well asto limit the hangover in operation of the control branches to facilitate the transmission of the subscribers talking current to the operator. I

Other objects and features of the invention will be brought out in the following detailed description when read in coniunctionwith the accompanying drawing, the singlefigure ofwhich shows schematically the circuits of an operators telephone set embodying one form I of the invention. i I A a The operators telephone circuit of the; drawing includes a voice transmitting branch TC and a voice receivingbranch RC coupled by the three-winding transformer TI to a two-Wire terminating section TS which is adapted for connection in any suitable manner to aselected telephone line. I I

The voice transmitting branch T C includes in order reading from "left to right, a telephone transmitter T with an associated energizing battery Bl, the transmitting.vario-losser TVL and the transmitting amplifier TA having its output connected to one winding l of the transformer TI. The voice receiving branch RC, whichhas its input connected to a second winding 2 of transformer Tl, includesthe frequency characteristic-correcting networks NI in its input followed by the receiving vario-losser RVL,

fiers 4 and 5 in series, shunted by a linear .1 'e-.

sistor'fi, connected in series with each side of '-the line section, and the linear resistors Land 8 shunted across the line section at the points indicated. The receivingwario-losser -RVL com,-

prises a bridge having a copper-oxide rectifier poled as indicated in each of its four arms, and

,. one diagonal (horizonta1) connected in series a with the linear resistors '9 and I9 across them- "ceiving branch 'RC in front of the receiving amplifier RA.

The transmitting amplifier *TA comprises a single four-electrode vacuum tube stage having its 'control'grid-cathode circuit connected to the secondary winding of the transformer T3 and its anode-cathode circuit'connected'to the wind- 'of its one-way transmission characteristialisto prevent incoming signals received over the connected telephone line from-a distant subscriber and impressed on transformer T1,; from passing over the voice transmitting branch -TC 'to" the input of the voice-operated control circuit CL to be described, and'causing false'operation of the latter to disable thevoice receiving ranch RC,

transmitting branch TC.

receiving amplifier RA comprises an input transformer T6, an output transformer T1 and a single four-electrode vacuum tube amplifying stage having a negative feedback circuit including the condenser 42 and the resistor 43 in series connected between the plate of the tube and the lower end of the input potentiometer 40. The receiving amplifier RA is used to supply the desired amount of receiving gain, the 'manually operated input potentiometer 1'- providing means for adjusting the amount of this gain. The cathode current of this amplifier is supplied from battery B3 through a high resistance il which serves as a filter to reject battery noise. The feedback circuit is used principally to adjust the output impedance of the'amplifierto desired value, but also serves to reduce unwanted high frequency response and increases the amplifier stability. 1

The volume limiter VL comprising two parallel-connected copperoxide rectifiers II and I2, connected in shunt with the voice receiving branch RC between transformer T1 and the re ceiver R, serves as a peak chopper to protect the operator from high amplitude signals.

The control circuit CL for the vario-lossers TVL and RVL has its input connected across the secondary winding of transformer T2 in the voice a hysteresis vario-losser HVL followed by an alternating current amplifier CA comprising an input transformer T5 and two resistance-condenser coupled four-electrode vacuum. tube stages used to obtain sufficient sensitivity for the control circuit. A negative feedback circuit including a series condenser ll, connected between the plate of the last stage tube and the cathode of the first stage tube, is used for improving the stability and modifying the frequency response of the control circuit. An input potentiometer 29 for amplifier CA is provided for adjusting the initial sensitivity of the control circuitCL.

Two independent detector circuits respectively including the copper-oxide detectors Di and D2, have their inputs connected by transformers T3 and T4, respectively, to the plate-cathode circult of the last stage tube of amplifier CA. The direct current amplifier A! consisting of.a single four-electrode amplifying vacuum tube has its control grid-cathode circuit connected directly to the output of the detector circuit including detector Di and its anode-cathode circuit including the battery B6 and the resistance-condenser network l8 in series, connected tothe transmitting vario-losser TVL across the mid-points of the secondary winding of transformerTZ and the primary winding of transformer T3. A direct current amplifier A2 comprising a. singlefourelectrode amplifying vacuum tube has its control grid-cathode circuit connected. directly to the output of the detector circuit including detector D2,'and its anode-cathode circuit including plate battery Bl, resistance-condenser network l9 and the resistor 20 in series connected across the vertical diagonal of the receiving -vario-1osser RVL.

The hysteresis vario-losser HVL comprising a network'c'onsisting oftwo series copper-oxide rectifiers l3 and I4, shunted by a linear resistor .4

It includes in its input y 15, in each side of the control circuit CL, and a linear resistor i6 shunted across the circuit between the rectifiers, is supplied with a variable control bias from the output of the direct current amplifier tube AZ by a direct connection 28 from the plate of that tube to the midpoint of tthe primary winding of the input transformer A network comprising a varistor 30, a condenser 31 and a linear resistor 32 in parallel, is connected in common to the output circuit of the detector DI and the control grid-cathode circuit of the direct current amplifier tube Al, and a similar network comprising the varistor 33, the condenser 3-! and the linear resistor 35 in parallel, is connected in common to the output circuit of the detector D2 and the control gridcathode circuit of the direct current amplifier tube A2. The varistors 3i! and 33 may each comprise a block of a material composed of silicon carbide crystals and an insulating binder (kaolin), commercially known as Thyrite,

which is disclosed in Patent No. 1,822,742, issued September 8, 1931 to K. B. McEachron.

Biasing voltages for the detector DI and the grids'of the direct current amplifier tube Al are provided by the voltage drops produced in the series resistors 2i to 23 by current from battery B4, and for detector D2 and the grids of amplifier tube A2 by the voltage drops produced in the series resistors 25 to 21 by current from battery The time constants of the vario-lossers TVL,

RVL and HVL are determined by the resistancecondenser combinations 33, 3|; 32 and 34, 35, in the grid circuits of the direct current amplifiers Al and A2 in a manner which will be described later. The resistance-condenser networks 36, 3?, 38 in amplifier CA are screen and cathode current limiting networks.

The manner in which the circuits of the operators telephone circuit accomplish the objects of the invention will be brought out in the following complete description of operation.

Normally, with no speech input to the control circuit CL, the direct current amplifier tubes Al and A2 are biased to cut-01f. The batteries B6 and B1 are smaller than the batteries B4 and B5, respectively. Therefore, with the tubes Al and A2.cut off (idle condition) there are no voltage drops produced in the resistors of 18 and i9, andwthe mid-tap on the primary winding of are pointed, making the rectifier resistance high so that TVL provides a normal high loss in TC and RVL a normal low loss in RC. Thus, with no speech input to the operators transmitter T, thereis a high loss in the transmitting voice branch. TC and .a lowloss between the input of the receiving amplifier RA in the voice receiving branchRC and the telephone line connected to the terminating section TS of the telephone set, so that room noise cominginto the transmitter is highly attenuated andspepch signals incoming over the connected line andimpressedby transformer. T I, on the voice receiving branch RC pass freelyltoLthereceiving amplifier RA in which they will be amplified and with a volume as limited by the peak volume limiter VL will be reproduc'ed by the telephone receiver R of the'operator';

A small'current also normally flows from the battery 135 through theresistance of I9, the'conductor 23, the primary vwindings of the .input transformer T of the control amplifier CA and the rectifiers I 3, I I .of the hysteresis variolosser HVL in the inputof the control circuit CL in a direction which is the opposite to that in which they are pointed. With this bias, these rectifiers have a relatively .high 'resistan ce, so that the vario-losser HVLnormally provides an appreciable loss in theinputof the control circult, which is made by proper selection of the circuit elements sufficient to prevent false operation'of the control circuit CL in response to sharp applied noises, such aswould be produced by the dropping of cords and plugs encountered at any manual switchboard, I

When the operator speaks, her speech Waves pass through the input transformer T2 to the transmitting losser TVL and the input of the control circuit CL. The portion passing into'the control circuit CL is. attenuated by the variolosser I-IVL and is'then impressed by the transformer T6 on the control amplifier CA which amplifies it in accordance with the gain setting of potentiometer 29. L

A portion of the amplified speech energy in the output of the amplifier CA is impressed by'the transformer T3 on the inputof the copper-oxide detector DI, and the other portion is impressed by the transformer T4 on the input of the copper-oxide detector D2. The rectified signal outputs of the detectors DI and D2 respectively produce positive potentials on the control grids of the direct current amplifier tubes Al and A2, causing plate current flow from batteriesBB and B1, respectively, through the copper-oxide rectifiers in the threevario-lossers TVL, RVL and HVL in the direction in which these rectifiers'are pointed.

The reverse biasingcurrent then flowing through the copper-oxide rectifiers l3, M of the vario-losser pad HVL in control circuit CL, reduces their series resistances to a very low value r so that the loss value of the pad HVL is effectivethe copper-oxide rectifiers, which in the set 0011- structed'was about 7 decibels less than before speech was applied. A very short pulse of :input will switch out this loss, increasing thesensitivity of the control circuit about '7 decibels, but this higher sensitivity will be maintained for only a short time (approximately 20milliseconds) after the input signal ceases. Thus, a sharp noise, such as that from a plug tip hitting the operators key shelf may increase the sensitivity momentarily, perhaps to operate the control circuit to interrupt incoming speech from the subscriber. However, noise conditions at the switchboard with which such sets are to be used are usually such that the background noise is rarely suf' ficient to prolong the false operation caused by clicks. Thus, the mutilation of received speech is relatively small even with extremely high room noise, e. 75 to 80 decibels above reference noise level.

The hysteresis vario-losser HVL is controlled from the direct current amplifier tube A2 which also controls the receiving vario-losser RVL. rather than from the amplifier Al controlling the transmitting vario-losser TVL, because of faster operation of the detector D2 associated with. the

I 3 former amplifier (thereisjno resistor. in series with the detector D2 corresponding-to the series resistor associated with the detector Di); If

' the control were j taken from the. direct current amplifier Al, the purpose of the series resistor 24 in series with its associated detector-DI would be defeated, and undue clipping of the outgoing speech"- would result. The association of the hysteresis loss pad with the receiving control branch provides a shorter hangover time in the hysteresis pad switched. Once the operators speech has operated the control circuit, the change in bias produced by the increased current flowing through the copper-oxide rectifiers in HVL reduces their series resistances so that the hysteresis loss is effectively removed and less input is required to keep the control circuit operated than was required to initially operate it, The increased biasing current flowing from the outputs of the amplifiers Al and A2 respectively through the series copper-oxide rectifiers of the transmitting'vario-losser TVL and the shunt copper-oxide rectifiers of the receiving vario-losser RVL 'while the operator continues to talk, by decreasing the resistance of the rectifiers, reduces theloss value of the former variolosserto a. very low value, and increases the loss value of the latter vario-losser to a high value. This effectively increases the gain in the voice transmitting branch TC for the operators outgoing voice currents which are amplified by amplifier TA and then pass through transformer TI and two-wire section TS tothe connected line, while eifectively'reducing the gain of the voice receiving branch RC so that thetransmission of sidetone energy through thelatter branch .is not increased.

While the operator talks, the condenser 3! (0.1 microfarad), shunting varistor 30 in the upper detector branch is charged through the large (10,000 ohms) resistor 24 and. the forward resistance of thcopper-oxide detector DI, while the condenser 35 (.075 microfarad) shunting the varistor 33 in the lowerv detector branch is charged through'the forward resistance of the copper-oxide oxide detector D2 only, thereby providing the different variable biases on the grids of the direct current amplifier tubes Al and A2 required to relatively adjust the rate of change of gain produced by the vario-lossers TVL and R-VL in the transmitter'and receiver branches in the desired manner.

When the operator ceasestalking and the input to the detectors DI and D2 in the'two control branches is removed, the voltages built up across the condensers 3| and 3-1 in the load circuits of the respective detectors DI and D2 are discharged through the associated thyrite vari tors 30 and 33, the discharges'being quite-rapid at first due to the low resistance of varistors caused by the relatively high voltage. As the voltages on the two condensers 3| and 34. de-

cay,'the resistances of the associated varistors 30 and 33 increase, retarding the rate of decay further. [Since the loss .of' each copper-oxide rectifier vario-losser padTV'L, RVL. and I-WLisa function of thecurrent through. it, and the current changes with time, the loss through each pad changeswith time. By properly choosing the voltage-resistance characteristics of thevaristors 30, 33, the correct, change ingaincan be jobtained. It is extremely important, to have ,this, rate of change in gain. right in both variorlossers 'IZVL and RVL, since the transmissionaround theside tone path involves both, vario-lossers, simulta.- neously. If one should have arapid change in gain while .the other had its gain changed slowly, there would be times when the sidetone trans? mission would beconsiderably in excess or or considerably less than the steadystate values, This would result in a sidetone transient which might reach ,a value 25 decibels greaterthan nor.- mal. This would beannoying to the operator since it would give a blasting effect. Further precautions are taken against this by intentionally slowing down theratefiof increase in gain of the voice transmitting branch TO to match better the more gradual reduction, in ain in thevoice receiving branch RC produced by thewaristor 33. This is accomplished by the use of the additional (10,000 ohms) resistor 24 V in series with the detector DI in the control branch controlling the transmitting vario-losser TVL. The 7 large resistor 32 shunting the varistor 3il'and the large resistor 35 shunting the varistor 33 are provided for protective purposes In the set constructed, these resistors had the values, 2 megohms and 1 megohm, respectively, the values of the associated condenser elements being as. given above in parenthesis. j

The operators telephone set in accordance with the invention, which was built; and t6$fied, had the following other approximate characteristics:

The voice transmitting branch was initially set at 4 decibels loss and the voice receiving branch at 6 decibels gain;

The control circuit CL was tuned to. maximum response at 300 cycles, the initial sensitivity being decibels less at 300 and 3000cycles, andthe sensitivity before switching out hysteresis pad HVL was made variable in 7 steps from 22-to 34 decibels below .001 watt at 800 cycles, measured at transmitter input, by proper design of the input potentiometer 25 for the control amplifier CA.

By proper selection of the values of the elements in the resistance-condenser combinations 36, 3'! and 38 in the grid circuits of thetwo tubes in the control amplifier CA, and the impedances of the circuits containing the vario-losser pads TVL, RVL and HVL, to provide the proper switching time constants, the hysteresis pad HVL was made to switch about '1 decibels loss in about milliseconds and .to release in, about milliseconds; the transmitter vario-losser pad 'IVL was made to switch about 27, decibels loss to provide a room noise sidetoneloss, of about decibels at 100p cycles (.13 decibels at 200 and 5000 cycles) the loss being switched in about 10 milliseconds and released in about 60 milliseconds; and the receiving vario-losser, pad RV'L was made to switch about 2'7 decibels loss, providing speech sidetone loss of 25 decibels at 1000 cycles and 6 decibelshigher speed sidetone loss at 5000 cycles, the loss being switched in about 8 milliseconds and released in about 50 milliseconds. The. switching and release times will vary with the. signal amplitudes. The. .above estimated times .are given toconvey some idea of the amplitude s,,involved.

An additional feature of the, set which was built and testedwasanincrease in the sidetone at highirequencies, so that the set did not. sound dead..to the operator. This was obtained by proper selection of the elements inthe feedback circuit of the receiving amplifier RA to adjust its frequency, response characteristic.

Various, modifications of the circuits illustrated and described above, which would be within the spirit and scopeflof. the invention, will occur to persons skilled in the art.

What is, claimed is:

1 A telephone set comprising line terminals, a voice signal transmitting circuit and a voice signal receiving circuit including amplifier means, connectedto said line terminals, a variolosser in each of said circuits, the vario-losser in the transmitting circuit being normally biased soas to introduce a substantial loss in that circuit and the vario-losser in the receiving circuit being normally biased so as to introduce a very small loss therein, a control circuit connected to said transmitting circuit, having two control branches each including asignal detector, rcsponsive to outgoing voice signals in said transmitting circuit to respectively vary the bias on and thus the loss ,of the vario-losser in said transmitting circuit and of the vario-losscr in said receiving circuit in. such manner as to increase the gain of the former circuit and to reduce the gain of thelatter circuit, and a network including a capacitor shunted by a varistor the resistance of which increases as the voltage across it decreasesin the detector load circuit of each control branch for independently controlling the time constants of the two control branches to control the rate of change in gain produced by said vario-lossers.

2. The telephone set of: claim 1, in which the detectorin each control branch is followed by a direct current amplifier the output current or which controls the loss value of a diileren: one of the vario-lossers in accordance with its amplitude, said capacitor and shunting varistor being connected in common to the output circuit of the detector and .the input circuit of the direct. current amplifier, the capacitor being charged by the detected voice signals and discharging through the ,varistor when the signals cease.

3. The telephone set of claim 1, in which the vario-lossers in said transmitting and receiving circuits include a plurality of copper-oxide rectifier elements, the detector in each of said control branches being followed by a direct current amplifier the output current of which is passed through the copper-oxide rectifiers in a different one of said varioelossers to control the resistance values of the rectifiers in accordance with the amplitude of said currents.

4. The telephone set of claim 1 in which the detector in each control branchis followed by a direct current amplifier the output current of which controls the loss value of a different one of said vario-lossers in accordance with its amplitude, said network being connected in common to the output circuit of the detector and the input circuit of the direct current amplifier, and cheat said control branches includes means for making ,the charging current to said capacitor therein different from the charging current to the capacitor in the other control branch.

5. TheteleDhQne SE1 .Qfnclaim l in which the vario-lossers in said transmitting and receiving circuits include a plurality of impedance elements having a non-linear resistance-current characteristic, the detector ineach of said control branches being followed by a direct current amplifier the output current of which is passed through said non-linear impedance elements in a difierent one of said vario-l'ossers so as'to cone trol the loss value thereof in the desired manner,

said network in eachcontrol branch being connected in common to the output circuit of the detector and the input circuit of the direct current amplifier, said control branch controlling the loss two control branches each including a signal detector followed by a direct current amplifier," re sponsive to outgoing signals in said transmitting circuit to control the loss values of the two variolossers, so as to effectively increase the gain of said transmitting circuit and reduce the gain of said receiving circuit, a third vario-losser in the input of said control circuit normally having a loss value sufiicient to prevent false operation 'of that control circuit by noise waves of certain types, andmeans responsive to the output current of one of said direct current amplifiers when voice signals are beingapplied to said control circuit to reduce the loss value of said third,

vario-losser and thereby increase the sensitivity of said control circuit. i y

7. An operators telephone set comprising a two-wire terminating section adapted for-con-' ments in said transmitting circuit, a second vario-losser including shunt non-linear impedance elements, in said receiving circuit, a control circuit connected to said transmitting circuit in front of the vario-losser therein, having two control branches each including a signal detector followed 'by a direct current amplifier, the a output circuit of one "of said direct current amplifiers beingconnected across the vario-losser in said transmittingcircuit so that its output current flows through said series non-linear impedance elements and theoutput'of the other direct current amplifier being connected across the vario-losser in saidreceiving circuit so that its output current flows through said shunt impedance elements, said direct current amplifiers being normally biased so that said first vario losser normally provide a high loss in said trans- .mitting circuit andsaidse'cond vario-lossernormally provides a low, loss in said receiving cir-' cuit, the poling of the non-linear impedance elements in the two vario-lossers with respect to the'outputs of the controlling direct current am-- plifiers being such that the increase in the flow of the output current through said non-linear impedance elements when the operator talks re duces the, loss in-the transmitting circuit and increases the loss in said receiving circuit, and ad-f ditional means to reduce sidetone transients when said operator talks including a network consisting of a capacitor shunted by an element having a negative resistance-current characteris-' tic connected across the detector output circuitin each control branch, of such values as to provide the proper relative time constants for the two control branches.

8. The telephone set of claim 7, in' which an additional series resistor is connected between the detector in the control branch controlling the loss of the vario-losser in said transmitting nection to a telephone line, a voice transmitting circuit including a telephonefitransmitter and'an amplifier and a voice receiving circuit including a telephone receiver and an amplifier, inductively coupled to said terminating section, a variocirc'uitgto' slow up the operation of that control branch with respect to the operationof the othercontrol branch.

losser including series non-linear impedance ele' s. BERGER. V 

